1. FIELD OF THE INVENTION
The present invention relates to a road surface detecting device for detecting the condition of a road surface on which a vehicle run.
2. DESCRIPTION OF THE PRIOR ART
When a vehicle runs on good roads streets or the like, it is desirable to absorb vibrations of small amplitude caused by the road surface. Absorbing such small amplitude vibrations is effected by decreasing the damping force of a suspension of the vehicle, contrastingly when a vehicle runs on bad roads such as mountain roads, it is desirable to damp vibrations of large amplitude from the road surface. Large amplitude vibrations are absorbed by increasing the damping force of the suspension of the vehicle.
In order to automatically control the damping force of the suspension in such a manner as described above, a detecting device for detecting the condition of the road surface on which a vehicle runs is required. A device for determining the road condition in accordance with the variation of vehicle speed has been proposed for this purpose. This device is described in U.S. Pat. No. 4,506,909, and senses the acceleration of the vehicle.
Since the variation of vehicle speed accurately corresponds to the road condition, the above described device exhibits excellent performance. However, the above described device has a problem. Namely, when a vehicle passes rough areas on a good road, such as joints of a road, the variation of vehicle speed is temporarily increased. This results in the above described device erroneously determining this condition as being a bad road condition and consequently increases the damping force of a suspension of the vehicle. Consequently, the vehicle becomes uncomfortable to ride in.